Use It Less, Lose It More.

It took a 12-day ride in space to figure out the underlying science, but the conclusion is very clear: Lack of resistance activates bone-destroying cells known as osteoclasts. The study, which was published in The FASEB Journal, analyzed bone marrow cells fed and maintained in specially designed bioreactors. One group remained on Earth while an identical group blasted into space aboard the European Space Agency's FOTON-M3 mission. After the flight ended, the orbiting group of cells displayed much greater osteoclast maturation and activity. Researchers estimate that an astronaut could lose 10% to 15% of his bone mass during a 6-month mission in space.

The Bigger Picture: Although you aren't likely to sign up for an assignment on the International Space Station, Earth's gravity can only do so much to help you maintain bone density. Sitting in front of the TV every night you might as well be floating in space. As this research points out, regular resistance training can build your bones as well as your muscles. The importance of this fact increases dramatically as we approach middle age.